Background on Re-Entry Permits
Lawful permanent residents (green card holders) who depart from the U.S. must reapply for admission to the U.S. upon return from a temporary visit abroad in the same manner as any other foreign citizen.
Lawful permanent residents bypass the general requirements for admission, but still must present a valid re-entry document to gain admission to the U.S. The Permanent Resident Card (Form I-551, the physical “green card”), may be used to re-enter the U.S. if seeking readmission after a temporary absence of less than 1 year.
If a permanent resident does not return to the U.S. within 1 year, she must obtain other documentation to re-enter the country. For absences beyond 1 year, a permanent resident may apply for a re-entry permit.